1. Field of the Invention
The present invention relates to a holographic memory and an optical information recording and reproducing apparatus using the holographic memory.
2. Description of the Related Art
Conventionally, a holographic memory system is known as a digital recording system using the principle of holography. The holographic memory system records and reproduces digital data to/from a memory medium of a photorefractive crystalline material such as lithium niobate (LiNbO3). The photorefractive effect is a phenomenon in which electric charges generated by photoexcitation move through crystals to thereby form a spatial electric field distribution, which couples with a primary electro-optical effect (i.e., Pockels effect) to change a refractive index of the crystals.
With regard to a ferro-electric crystal which exhibits the photorefractive effect, a refractive index change normally responds to an optical input pattern of 1000 or more lines per 1 mm, and the effect appears in real-time at a response speed of the order of micro-second to second, depending upon a material of the crystal. Therefore, such crystal has been studied as a real time hologram medium, which does not need developing, with respect to various applications.
The holographic memory system can record and reproduce data in a two-dimensional plane page unit, and perform multiple recording with the use of a plurality of pages. The volume holographic memory enables three-dimensional recording with the memory medium being of three-dimensional configuration such as a rectangular parallelepiped. The volume holographic memory is a kind of Fourier transform holograms. Data is recorded in a dispersed manner by unitary image pages in a three-dimensional space of the memory. An overview of the holographic memory system will be described hereinbelow with reference to FIG. 1.
In FIG. 1, an encoder 25 converts digital data to be recorded in a volume holographic memory 1, into a dot pattern image of light and darkness in a plane, and rearranges the image in a data arrangement of, for example, 640 bits in a line and 480 bits in a row to generate a unitary-page sequence data. The unitary-page sequence data is supplied to an SLM (Spatial Light Modulator) 12 such as a panel of a transmission type Thin Film Transistor Liquid Crystal Display (hereinafter, referred to as xe2x80x9cTFT-LCDxe2x80x9d or xe2x80x9cLCDxe2x80x9d).
The SLM 12 performs a modulation processing by the processing unit of 640 pixels in a line and 480 pixels in a row, which corresponds to a unitary page. More particularly, the SLM 12 performs light modulation of a light beam or a source beam into an on/off signal of spatial light, corresponding to the unitary-page sequence data from the encoder 25. The modulated source beam or a signal light beam (hereinafter, referred to simply as xe2x80x9csignal beamxe2x80x9d) is conducted to a lens 13. More specifically, the SLM 12 passes therethrough the source beam in response to the Boolean value xe2x80x9c1xe2x80x9d of the unitary-page sequence data, which is an electric signal, and shuts off the source beam in response to the Boolean value xe2x80x9c0xe2x80x9d to thereby achieve electro-optical conversion in accordance with the contents of respective bits in the unitary page data. Accordingly, the signal beam of the unitary page sequence is generated by modulation of the source beam.
The signal beam is incident upon the volume holographic memory 1 through the lens 13. In addition to the signal beam, a reference light beam (hereinafter, referred to simply as xe2x80x9creference beamxe2x80x9d) is incident upon the volume holographic memory 1 at an angle xcex2 (hereinafter, referred to as xe2x80x9cincident angle xcex2xe2x80x9d) relative to a predetermined reference line perpendicular to an optical axis of the signal beam.
The signal beam and the reference beam interfere with each other within the volume holographic memory 1, and the resulting interference fringe is stored as a refractive index grating within the volume holographic memory 1, whereby recording of data is effected. Also, recording of three-dimensional data is made possible by angular-multiplexed recording of a plurality of two-dimensional plane data with variance of the incident angle xcex2.
When reproducing the recorded data from the volume holographic memory 1, only the reference beam is made incident upon the volume holographic memory 1 at the same incident angle xcex2 as at the time of recording toward the center of a region where the signal beam and the reference beam intersect with each other. That is, reproducing the recorded data is different from recording data, in that the signal beam is not made to be incident. Therefore, diffracted light from the interference fringe recorded in the volume holographic memory 1 is conducted to a CCD (Charge Coupled Device) 22 in a light detector through a lens 21. The CCD 22 converts light and dark patterns of the incident beam into variations in intensity of an electric signal to output to a decoder 26 an analog electric signal having a level corresponding to brightness of the incident beam. The decoder 26 compares the analog signal with a predetermined amplitude (i.e., slice level) to reproduce data consisting of the corresponding xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d.
Because recording is performed in a two-dimensional plane data sequence within the volume holographic memory as described above, the incident angle xcex2 of the reference beam is varied to enable the angular multiplexed recording. That is, the incident angle xcex2 of the reference beam is varied to enable of defining a plurality of two-dimensional planes wherein the plane is a unit of recording, within the volume holographic memory. Therefore, three-dimensional recording can be achieved. An example of angular multiplexed recording is described in Japanese Unexamined Patent Publication Kokai Nos. H2-142979 and H10-97174.
In a case where a volume holographic memory is employed as a dismountable large capacity recording medium, a position, at which the memory is mounted on an optical information recording and reproducing apparatus, affects a positional relationship between a CCD photodetector and a reproduced image. Therefore, the position, at which the volume holographic memory is mounted, is greatly related to the quality of reproduction signal by a CCD. Thus, a part of image data to be recorded is conventionally used for adjustment of position. However, it has been required that an optical strain, a displacement of a signal image, and the like occurs between a spatial light modulator to a CCD image element, which are generated by a volume holographic memory when the volume holographic memory is newly mounted, should be settled within a predetermined specified value.
Further, when the volume holographic memory in which data is recorded by a certain optical information recording and reproducing apparatus is reproduced by other optical information recording and reproducing apparatuses, a reproduced image is much deformed by a position dispersion between the volume holographic memory and the CCD image element at the time of recording and reproduction. Therefore, it is necessary to adjust the position of the CCD image element or the volume holographic memory to a substantial extent. Thus, the conventional volume holographic memory had a drawback in interchangeability.
It is an object of the present invention to provide an optical information recording and reproducing apparatus having a holographic memory, which is capable of recording an interference pattern in high density and retains interchangeability.
According to the present invention, there is provided a holographic memory made of a photorefractive material, the holographic memory comprises: a nonvolatile refractive index grating corresponding to a three-dimensional light interference pattern of a coherent reference beam and a coherent signal beam, the coherent signal beam being modulated in accordance with positioning images.
According to another aspect of the present invention, each of the positioning images includes a continuous image or scattered images disposed symmetrically with respect to a point.
According to still another aspect of the present invention, the positioning images are scattered and disposed every page of a predetermined cycle.
According to further another aspect of the present invention, the positioning images are intensively disposed in a predetermined fixing region and are disposed every page of a predetermined cycle.
According to another aspect of the present invention, the holographic memory comprises a rotating uniaxial crystal having an optical axis, the optical axis being in parallel to a symmetrical axis of rotation of the rotating uniaxial crystal.
According to still another aspect of the present invention, the holographic memory comprises a rectangular parallelepiped uniaxial photorefractive crystal having an optical axis, the optical axis being in parallel to a plane of the rotating uniaxial crystal.
According to the present invention, there is provided an optical information recording and reproducing apparatus, the apparatus comprises: a holographic memory made of a photorefractive crystalline material; a support portion for detachably supporting the holographic memory; a reference beam supplying portion for supplying a coherent reference beam of a first wavelength incident on the holographic memory; a signal beam generating portion for supplying a coherent signal beam of the first wavelength, which is modulated in accordance with image data, incident on the holographic memory, intersecting therein the signal beam with the reference beam, and for generating a three-dimensional light interference pattern of the signal beam and the reference beam; a detecting portion for detecting a diffracted light from a refractive index grating of the light interference pattern in the holographic memory, due to irradiation of the reference beam; and a medium position adjusting portion for moving a position of the support portion, which supports the holographic memory, in accordance with a signal corresponding to positioning images from the detection portion, wherein the holographic memory includes a nonvolatile refractive index grating corresponding to a three-dimensional light interference pattern of a coherent reference beam and a coherent signal beam, the coherent signal beam being modulated in accordance with positioning images.
According to another aspect of the present invention, the medium position adjusting portion is provided with a mechanism for performing parallel motion of the holographic memory in a direction along an optical axis of an optical path of the signal beam from the signal beam generating portion and in two directions perpendicular to the optical axis of the optical path included in a meridional plane and a sagittal plane, respectively, and for performing rotation of the holographic memory around the optical axis of the optical path and the two directions.
According to another aspect of the present invention, the apparatus further comprises a detected position adjusting portion for shifting a position of the detecting portion in accordance with a signal corresponding to positioning images supplied from the detecting portion.
According to still another aspect of the present invention, the detected position adjusting portion is provided with a mechanism for performing parallel motion of a light receiving surface of the detecting portion in the direction along the optical axis of the optical path of the signal beam from the signal beam generating portion and in two directions perpendicular to the optical axis of the optical path included in the meridional plane and the sagittal plane, respectively, and for performing rotation of the surface around the optical axis of the optical path and around the two directions.
According to another aspect of the present invention, the apparatus further comprises a spherical-wave reference-beam supplying portion for making a coherent reference beam including a convergent spherical wave of a first wavelength incident upon the holographic memory.
According to further another aspect of the present invention, the spherical-wave reference-beam supplying portion is provided with a mechanism for performing rotation of a coherent reference beam including the convergent spherical wave of the first wavelength about the holographic memory.
According to still another aspect of the present invention, the holographic memory includes a predetermined fixing region, in which the positioning images are intensively disposed and arranged every page of a predetermined cycle; and further comprises: a positioning reference-beam supplying portion for making a coherent reference beam of the first wavelength incident only on the fixing region; and a positioning detecting portion for detecting a diffracted light from the refractive index grating of the light interference pattern in the fixing region, due to irradiation from the positioning reference-beam supplying portion.
According to another aspect of the present invention, the apparatus further comprises a gate beam supplying portion for supplying a gate beam incident on the holographic memory, by which a gate beam of a second wavelength for increasing photosensitivity of the holographic memory and for one of activating and deactivating the refractive index grating depending upon the existence or non-existence of the light interference pattern.
According to another aspect of the present invention, the gate beam supplying portion includes a superluminescent diode.
According to another aspect of the present invention, the gate beam supplying portion includes means for restricting irradiation of the gate beam on a region where the signal beam and the reference beam intersect with each other.